Suspended spans in subsea pipelines are typically constructed in ways to mitigate the irregularities in seabed bathymetry. A subsea pipeline span may be subjected to motions due to currents that produce a phenomenon commonly referred to as vortex induced vibration (VIV). The motions could result in high cycle fatigue damage that may potentially reduce the effective life of the pipeline. Some pipelines operating in deep waters are not affected by surface wave effects but are affected by VIVs.
Reducing the length of a pipeline span may accomplish sufficient mitigation of the VIV effects provided that the natural frequency of the pipeline span is away from the shedding frequency of the VIV so that no resonant vibration can take place. Pipeline span length reduction may be accomplished by re-routing the line along a path associated with shorter spans or by supporting the span using various methods. These span length reduction attempts are in many cases very costly or practically unfeasible.
Helical strakes are widely used to mitigate VIV as they can generally accomplish significant pipeline response reductions with no requirements for pipeline span shortening. Due to the many uncertainties involved in VIV response prediction, the engineering methodologies currently available tend to be conservative and decisions for the provision of strakes over significant extensions of pipeline lengths are not unusual. However, use of helical strakes increases project costs for procurement, fabrication, inspection, transportation, and installation, as well as detrimentally affecting the associated project schedule and risk.
One important factor inherent in pipeline VIV response is the low amount of damping of the system. Typically, the pipelines have small amounts of damping because (a) the damping capacity of the pipeline itself (structural damping) is quite limited, (b) the pipeline is in contact with the soil only at the span ends which significantly limits the soil contribution to the damping of the system, and (c) under the effects of VIV, usually referred to as “lock-in,” there is no hydrodynamic damping available. Hence, the VIV response, which fundamentally is resonant-like, is not adequately mitigated by damping. This lack of damping is a major limitation, especially because resonant-like responses are very sensitive to and can be significantly reduced by increases in system damping.